I am having an issue finding any documentation on calculations for 50hz systems. Here is my example:

If i have to design a system around 30kw heaters supplied with 480 volts from a 3phase service.

Purely resistive loads typically have a PF of 1. So 30,000\480 x 1.732 = 36 amps

What would the current draw be for 50hz 480v 3phase? What is the formula to solve for this?

I read something somewhere, which i cant for the life of me find, about a volt frequency ratio; any help would be appreciated.

Thank you in advance.

Frequency is not a factor in this calculation. P = U * I * SQRT(3)

You may be thinking of the motor formulas for running a 50 hz motor at 60 hz or any other voltage than what the motor is wound for.

Are the heaters actually three phase or are they single phase heaters and just the service is three phase?

This is my concern. In america we assume with our calculations 60hz. If a system is designed and spec'd at 60hz what happens when you install said system into a facility who's power supply is rated at 50hz, for example almost the rest of the world...

Does this affect only motors or both motors and purely resistive loads?

So I found this equation X(l) = 2pi x F x L where f is the frequency and l is henries. 2 is the constant for a single phase system i confirmed this using a known value for X(l)

So now lets move to a 3phase system. X(l) = 1.732 x pi x F x L

Lets use my question above as the example. 30kw \ 480 x 1.732 = 36A

480 x 1.732 \ 36 = 23

So lets plug in 23 for X(l) so 23 = 1.732 pi x F x L

23 = 5.43848 x 60 x L

23 = 326.3088 x L

L = 23 \ 326.3088

L = .070485381

Now we have the all of the variables solved for lets check if this makes sense.

Lets cut the frequency in half. X(l) = 5.43848 x 30 x .070485381

X(l) = 11.5 so lets this in.


480 x 1.732 \ 11.5 = 72A

We have doubled the current by reducing the frequency by half, this makes sense to me.

So now lets plug in 50hz

X(l) = 5.43848 x 50 x .070485381

X(l) = 19.17

480 x 1.732 \ 19.17 = 43 amps.

This again makes sense to me

However my concern is if this affects a system designed for 60hz put in service at 50hz, again it will be for a purely resistive load (heaters)

It is to my understanding it will work but it will draw more current, and this is the math ive used to solve for it.

Inductuve resistance has nothing to do with simple resistance, you are going in completely the wrong direction. As the frequency is important for inductive and capacitive resistance, it is totally of no influence for resistive loads,

Ok so a change in frequency has no influence on a purely resistive load. that was my main concern. Thanks for the input!

I know 480 isnt a standard, I typically see 380, 575, and 600V for our foreign customers. I was just using a random voltage for the calculation.

Thanks again to all who responded.

As Mr. David said , purely resistive load has no concern with frequency. Out of your question and just to support ideas:

Inductive Load:
----------------------
- For 60Hz inductive load to be run on 50Hz multiply the original Voltage level by the ratio of 50/60 to get the new reduced voltage .
- For 50Hz inductive load to be run on 60Hz multiply the original Voltage level by the ratio of 60/50 to get the new increased voltage level.

Capacitive Load:
------------------------
- For 60Hz Capacitive load to be run on 50Hz multiply the original Voltage level by the ratio of 60/50 to get the new required increased voltage .
- For 50Hz inductive load to be run on 60Hz multiply the original Voltage level by the ratio of 50/60 to get the new reduced voltage level.